Conventionally, halogen lamps and HID (high-intensity discharge) lamps were the mainstream of light sources of vehicle lamps, in particular, headlights. However, in recent years, to replace such lamps, vehicle lamps using a semiconductor light source such as an LED(s) (light-emitting diode(s)) or a laser diode have come to be developed.
Vehicle lamps using a semiconductor light source are required to have a function of detecting an open-circuit abnormality due to open-circuit destruction of the semiconductor light source, coming-off of a harness, disconnection of an interconnection, or the like and notifying the vehicle side of it. FIGS. 1A and 1B are circuit diagrams of vehicle lamps 1r and 1s that are equipped with a lighting circuit having an open-circuit abnormality detecting function. These circuits are ones that the present inventor studied before the conception of the present invention and should not be considered to be part of the prior art.
A lighting circuit 10r shown in FIG. 1A is equipped with a buck converter 20 and an open-circuit detection circuit 30r. The lighting circuit 10r is supplied with a voltage VBAT from a battery 4 via a switch 6. The buck converter 20 lowers the voltage VBAT and supplies a resulting output voltage VOUT to a light source 2. The buck converter 20 is feedback-controlled by a converter controller (not shown) so that a drive current IDRV flowing through the light source 2 comes close to a target value IREF that governs a target light quantity of the light source 2.
The open-circuit detection circuit 30r shown in FIG. 1A is equipped with a sense resistor RS for current detection and a comparator 32r. The sense resistor RS is inserted in the path of the drive current IDRV, and a voltage drop (current detection signal) VIS which is proportional to the drive current IDRV develops across the sense resistor RS. The comparator 32r compares the current detection signal VIS with a threshold voltage VTH.
When the vehicle lamp 1r shown in FIG. 1A is normal, a normal drive current IDRV flows through the sense resistor RS and a voltage drop VIS that is larger than the threshold voltage VTH occurs. On the other hand, if an open-circuit abnormality has occurred, no drive current IDRV flows, as a result of which the voltage drop VIS becomes substantially equal to 0 V and hence lower than the threshold voltage VTH. Therefore, the output signal of the comparator 32r has a first level (e.g., high level) indicating that the vehicle lamp 1r is normal when VIS>VTH and a second level (e.g., low level) indicating occurrence of an open-circuit abnormality when VIS<VTH.
The open-circuit detection circuit 30s shown in FIG. 1B is equipped with resistors R11 and R12 and a comparator 32s. The resistors R11 and R12 divide the output voltage VOUT of the buck comparator 20. The comparator 32s compares a divisional output voltage (voltage detection signal) VVS with a threshold voltage VTH.
When the vehicle lamp 1s shown in FIG. 1B is normal, the output voltage VOUT is feedback-controlled to a voltage level that is most suitable for supply of a target current IREF to the light source 2. If an open-circuit abnormality has occurred, no drive current IDRV flows and the controller of the buck converter 20 increases the switching duty ratio so that the drive current IDRV comes closer to the target value IDRV, as a result of which the output voltage VOUT increases. As a result, the voltage detection signal VVS exceeds the threshold voltage VTH.
Therefore, the output signal of the comparator 32s has a first level (e.g., high level) indicating that the vehicle lamp 1s is normal when VVS<VTH and a second level (e.g., low level) indicating occurrence of an open-circuit abnormality when VVS>VTH.